Advertisement

Advertisement

Self-propelling droplets creep towards heat to cool microchips

Heading for the heat

Aminart/Getty

By Lakshmi Supriya

Cold water is in the hot seat. Usually, cool water moves away from hot areas – but make the droplets small enough and surprisingly, the opposite happens. The effect may help us cool electronic devices, with tiny droplets of water propelling themselves to the hotter areas before evaporating and cooling the surface.

Fluids tend to move when there is an imbalance of surface tension – the force that helps you blow soap bubbles. Temperature can cause surface tension to change, with fluids generally flowing away from warmth.

But get down to the nanoscale and the effect is reversed. Using computer simulations, Suman Chakraborty at the Indian Institute of Technology in Kharagpur and his colleagues have found that nanometre-sized water droplets on water-repellant surfaces move towards the heat.

Advertisement

“This traditional manifestation of droplet motion from high temperature to low temperature can be reversed,” says Chakraborty.

The effect is down to an increase in Van der Waals forces, which attract molecules in the droplet towards molecules in the surface on which it is resting. In this case, the enhanced force pulls water molecules out of the droplet, causing them to evaporate. Although this force is present both on the hot and cold side of a droplet, the molecules on the hot side are more energetic so they can evaporate much faster.

As they evaporate, there’s a lower concentration of molecules on the warmer side, so those from the cooler side start moving toward the heat, Chakraborty says.

That localised cooling, which increases the local surface tension where the droplet touches the surface, causing the droplet to slide along towards the hotter region.

This phenomenon may not occur at larger scales, says Sanjeev Kumar Gupta at the Indian Institute of Science in Bangalore. He says the temperature gradients studied are extremely large – any practical applications would need lower temperature gradients and drops that are at least micrometre-sized, because they are easier to make than nanoscale drops.

As microchips are made smaller and smaller, yet still transfer large amounts of power, they will need to be cooled. Exploiting this property of water may be one way to do this, but it would have to be in a separate micro-channel or under a polymer layer so as to protect the electronics.

For now, this is just a simulation. Chakraborty says he and his team may do some larger-scale experiments in the future to see how large a droplet can be and still show this behaviour.